DESCRIPTION: Dr. Mann has identified a 6 kDa N-terminal domain of human lactoferrin (LF), derived from neutrophil lysosomes, that has both glycosaminoglycan (GAG)-binding and antibacterial activities. This N-terminal, cationic peptide of LF can be liberated from the 83 kDa LF molecule by the action of certain proteases. The antibacterial action of this peptide appears to be distinct from that displayed by LF, which has previously been attributed to its iron-binding activity. Dr. Mann has evidence that this 6 kDa peptide can be found in tissues and can be generated by activated monocytes and neutrophils. Lysosomal cathepsin D is a candidate for processing LF and through its proteolytic action can generate the 6 kDa peptide. Moreover, a synthetic 33-mer peptide can mimic the action of the 6 kDa peptide. Additionally, at sub-lethal concentrations, the peptide can render E. coli sensitive to sub-MIC levels of a hydrophobic antibiotic. This raises the interesting question of whether the membrane-damaging activity of the peptide could be exploited to sensitize bacteria to hydrophobic antibiotics that have difficulty penetrating the outer membrane permeability barrier. The central hypothesis of this proposal is that the 6 kDa peptide is a candidate for combating bacterial pathogens during infection. In the present proposal Dr Mann will examine the in vivo occurrence of this novel peptide, establish the cellular and proteolytic events responsible for its activity, and determine its mechanism of antibacterial action. The results from these experiments should enhance our knowledge regarding host defense systems at mucosal surfaces.